Crucible furnace



Aug. 26, 1941 K. D. HOKE CRUCIBLE FURNACE 4 Sheets-Sheet 1 Filed Dec. 9,1939 INVENTOR. ffennelh Dav d Hoke ATTORNEY.

Aug. 26, 1941. K. D. HOKE CRUCIBLE FURNACE Filed Dec. 9, 1959 4Sheets-Sheet 2 MN NN Kennl'lz' Dav INVENTOR. idHoke BY 7 A ORNEY.

Aug. 26, 1941. K. D. HOKE ,CRUCIBLE 'FURNACE Filed Dec. 9, 1939 4Sheets-Sheet 4 Nbx N INVENTOR. KI /1710272 David Hoke BY ATTORNEY.

Patented Aug. 26, 1941 CRUCIBLE FURNACE Kenneth David Hoke, Chicago,111., assignor to Fisher Furnace Company, Chicago, 111., a corporationof Illinois Application December 9, 1939, Serial No. 308,419

14 Claims. ,(01. 2263-14) This invention relates to furnaces and themethod of operating the same, and more particularly to oil fired or gasfired crucible furnaces.

Itis common practice among foundrymen engaged in the manufacture ofsmall castings from high melting point metals and alloys, to melt thecharge in a crucible placed in a furnace. These furnaces are generallycheaply constructed, with the result that not only are the fuel costshigh because of excessive consumption, but the furnace temperatures arerelatively low, requiring long periods for melting, thereby reducing thecapacity of the furnace and causing, high metal losses by permitting themetal to becorne gassed.

One of the objects of this invention is to provide 11 a crucible furnacein which high furnace temperatures can be quickly attained and in whichthe desired temperature can be readily retained.

Another object of the invention is to provide a crucible furnace forburning oil efficiently.

A third object of the invention is to provide a crucible furnace inwhich the waste heat in the combustion gases can be utilized forpreheating the oil and air prior to entering the burner nozzle.

Still another object of the invention is to provide a crucible furnaceof unitary and compact construction. v

A further object of the invention is to provide an improved carriage andsupporting device for the furnace cover.

Other objects of the invention will become apparent from the followingdetailed description and accompanying drawings of which Figure 1 is aplan view of a furnace in accordance with the invention;

Figure 2 is a vertical cross-sectional view taken on the line 22 ofFigure 1;

Figure 3 is a vertical view partly in cross-section, taken on the line3--3 of Figure 1;

Figure 4 is another vertical cross-sectional view taken on line i:l ofFigure 1; and

Figure 5 is an enlarged fragmentary, vertical cross-sectional view ofone side of the carriage for supporting the cover.

Referring to the drawings, the numeral l indicates generally a furnaceadapted to be supported above the ground by legs 3. The legs 3 arepreferably cast iron angle bars, four in number bolted to a circularsteel plate 5 on the bottom of the furnace.

The furnace is built in two main sections, the crucible heating sectionindicated generally by the numeral l and the air preheating sectionindicated generally by the numeral 9. As shown in Ill- ' the chamber isabout 22.5 inches.

the drawing (Figure 1), the crucible heating section 1 is cylindrical inshape, The outermostwall I] is a cylindrical steel shell approximately32 inches in diameter and 28 inches high.

Welded to the lower edge of the shell H is a circular angle I3 with oneside thereof extending horizontally inwardly to provide a surface towhich the bottom plate 5 can be welded.

Supported by the angle I3 is the fire brick ll. This brick may be madeof cheaper material than the remaining brick with which the furnace islined and is somewhat smaller in diameter than the shell l l. The brickH), which is approximately four inches thick and made of siliconcarbide, is adapted to fit into a space in brick I! and form a portionof the bottom lining of the furnace. Brick l9 is provided with apassageway or port 2| through which slag or metal can be withdrawn fromthe furnace in the event the crucible breaks.

- 1 The plate 5 is provided with a hole 22 immediately below the, port2] in order to permit the slag to run out thebottom of the furnace. cThe interior side walls 23 of the crucible heating chamber are also madeof silicon carbide i brick. The side wall is circular and formed fromcircular, semi-circularor arcuate sections of brick about two inchesthick. The inside diameter of the crucible heating chamber is 19 inchesat the bottom and 17% inches at the top. The depth of The brick in thelower half of the side wall is tongued on the upper edge and the'brickin the upper half is grooved onthe lower edge in order to make .a tightvfit. The bricks 25 on the upper half of the side wall are inclinedslightly inwardly toward the top in order to constrict the space betweenthe upper part of the wall and thecrucible. This construction tends tokeep the hot furnace gases near the bottom of the. crucible where themost intense heat is desired.

In building up the interior. side wall, the silicon carbide brick isspaced from the steel shell which forms the outer wall. The space.between the steel shell and the side wall brick is filled with fire-clay21, rock wool or other refractory insulation in order to prevent escapeof. heat from the furnace. The insulation is approximately four inchesthick at the lower portion and slightly thicker toward the top.

The top surface of the side wall and fire clay is overlaid with flatsilicon carbide brick 29 which provides a surface on which the lid .il,about 24 inches in diameter and 3% inches thick, can rest. The lid' 31has an opening 3-2 centrally thereof. Centrally located in the heatingchamber 33 and resting on the floor brick H is the crucible block 35upon which the crucible 31 is adapted to sit when it is being heated.The block 35 is also constructed of silicon carbide or other heatrefractory material and is approximately five inches high and somewhatsmaller in diameter than the chamber.

The air preheating section 9 has an outer U- shaped steel shell 33 whichis welded to shell I l at 45 and 3. The shell 39 is approximately 19inches wide. A U-shaped angle 45 is welded to the bottom edge of theshell 39 with one side 41 extending horizontally inwardly. The angle 45forms a surface to which is welded bottom plate 49.

Supported within the shell 39, immediately above the angle 25 and weldedthereto, is the annular steel setting 55 open at its outer edge. Aheating coil 53 is nested in the annular space of the setting 5!adjacent the vertical wall 55 thereof.

An annular enclosed steel structure 51 rests on the setting 5! and ispreferably welded thereto. The outer wall 59 of the structure 51 isspaced from the shell 39 and extends to the top of the furnace. Theannular space of the structure 5'! is provided with alternate spacedbaflies 50, one set of bafiies being welded to the wall 59 and extendinghorizontally more than halfway to wall 6| and the alternate bafllesbeing welded to wall SI and extending horizontally more than halfway towall 59. The annular structure 51 comprises the air preheater. An inlet63 is provided at the upper end thereof for air induction. An outlet 65is provided at the bottom of the preheater for withdrawal of airtherefrom. The outside diameter of the annular structure 51 isapproximately 17 inches and the diameter of the inside wall isapproximately 8 inches.

The space between the shell 39 and the outer wall 59 of the airpreheater is filled with fireclay, rock wool or other insulation 66about one inch thick. The space between the wall 55 of the setting 5!and the shell 39 is likewise filled with insulation.

The central portion of the setting El and air preheater 5'! is linedwith silicon carbide brick E! about one inch thick which may becircular, semi-circular or arcuate shapes adapted to form a circularwall when fitted together. As shown, three tiers of silicon carbide areplaced upon each other in order to form a cylindrical flue 69 throughwhich combustion gases from the crucible heating chamber pass beforethey are eX- hausted to the atmosphere. The space H! between thepreheater wall and silicon carbide brick lining is filled with siliconcarbide granules. The upper end of the fiue 69 is connected by ahorizontal passageway ll with the upper portion of the crucible heatingchamber. The passageway H is built of silicon carbide brick in order towithstand the high temperature of the furnace gases passingtherethrough. As shown in Figure 2, the bricks l3 forming the bottom ofthe passageway H are flat on the under side and taper graduallyupwardly, in the direction of the crucible chamber. These bricks alsotaper gradually outwardly in the direction of the crucible chamber sothat the passageway is wider at its junction with the crucible chamberthan it is at its juncion with the fiue 59. The width of the passagewayat the former junction is indicated by the dotted walls 15 and 11(Figure 3) and the width of the passageway at the latter junction isindicated by the Walls 19 and 8|.

The side walls 19 and 84 of the passageway H are formed from rectangularshape bricks stood on end against the sides of the floor bricks. Theroof of the passageway H is formed of arcuate shaped bricks 83 the sidesof which are tapered to the same degree as the floor bricks. The roofbricks are adapted to rest on the top of the side wall bricks. Thepassageway is approximately twelve inches long, three and one-halfinches high and five inches wide at its junction with the air preheater,and seven inches wide and two and one-half inches high at its junctionwith the crucible chamber.

The upper tier 25 of brick forming the interior wall of the cruciblechamber has an opening cut out to receive the end bricks $5 and 8'5.Likewise, the air preheater structure 5'! has a section cut out at thetop to accommodate the opposite end of the passageway. The floor bricks13 of the passageway are supported by the upper edge of the flue brick89, the top plate 9| of the air preheater, the horizontal projection 93of the shell II and the upper edge of brick 25.

Immediately overlying the roof of the passageway and supported at itsedges by the fire clay 66 is the thin fire brick 95. This brick ispreferably made in sections fitted together but may be in one piece. Thebrick 95 forms the top of the preheater section.

Referring more particularly to Figures 1 and 4, it will be seen that anoil tank 91 is placed in the space between the shell H, shell 39 and theouter wall 59 of the air preheater. In Figure 4 the insulation has beenpurposely omitted in order to more clearly show the tank. The lower endof the tank is connected by means of pipe 99 to the bottom of theheating coil and the upper end of the tank is connected by means of pipeIt?! to the top of the heating coil. Fresh oil is pumped to the lowerportion of the tank through pipe Hi3 which enters the top of the tankand extends Vertically downward to a level adjacent the bottom of thetank. Hot oil is withdrawn from the top of the tank through the lineI05. The line I55 connects from below at the point lil'l with the burnernozzle I09 equipped with a conventional control valve H9. The nozzle isset in an enclosed steel casing l l I which is in turn welded orotherwise fastened to the square steel neck I I2 welded or otherwisefastened to shell H at approximately the level of the crucible block 35.The neck H2 is lined with fire brick and has a central passageway H3frustro-conical in shape tapering from a larger outside diameter to asmaller inside diameter. A hollow casting H4 having a cylindrical portH5 at its innermost end is adapted to fit in the passageway H3. The tipH5 of the burner nozzle extends into the port H5. A frustro-conicalpassageway l I! forming the combustion chamber opens tangentially in thelower portion of the crucible chamber at the level of the crucible block35. This passageway is coincidental with the passageway H3. It will beseen that the diameter of the passageway H? at the line of meeting withthe inside edge of casting H4 is substantially the same as the diameterof the part H5.

A duct H8, three to four inches in diameter, connects the outlet 55 ofthe air preheater with the casing H I so that hot air can be admitted tothe space surrounding the burner nozzle. The only air which enters theburner opening comes through the duct H3 since the casing H l issubstantially air-tight. The handle H9 on the casing III is connected toa'damper Imadapted' toregulate'the amount of air admitted to thecombustion chamber.

The crucible chamber lid 3I','made of silicon carbide is bound by twosemi-circular steel bands IZIand I22 having flanges I23 and I25 on eachend provided with holes through which bolts lI2'I are adapted to bepassed to hold the bands securely around the cover.

A pair of horizontal steel tracks I29 and I3I are welded to the shell II adjacent the top.thereof and extend rearwardly along the U-shapedshell 39, spaced a short distance laterally thereof. A cross-bar I33 iswelded to each of the tracks adjacent their rear ends I35 and I31 andalso welded at its center to theshell39, thereby forming support for therear end of the tracks.

Slidably mounted on the tracks is a carriage generally indicated by thenumeral I39 (Figure 3). The carriage is constructed of two arms MI andI43 mounted on tracks I29 and I3I respectively, by means of flangedwheels I45, I41, I49 and II. The wheels I45 and I49' are rotatablymounted on a shaft I53 which is in turn rigidly mountedv between armsMI- and I43. Wheels 54? and I5I are rotatably mounted on pins I55 and I57 which are in turn rigidlyfastened to the lower ends of the arms MIand 543, respectively. The distance between wheels I45 and I4! andbetween I49 and i'5I is such that the track will fit snugly between themwhen the armsI iI and I43 are in substantially vertical position.

Angles B and ISI are welded to the arms I4! and I33, respectivelyadjacent the upper ends thereof. The angles'have holesbored therethroughthrough which are adapted to pass the upper ends of rods I63 and Irespectively. The upper ends of the rods I33 and" I35 are threaded inorder to receive nuts I51 and I39. The lower ends of the rods I53 andIE5 are weldedat HI and 573 respectively to the :band I2I surroundingthe lid 3i The lid Si is additionally fastened to the carriage by meansof the spaced rods I35, Ill and I73 welded'at. one end to the shaft I53and at the other end inserted in a hole inand welded to the band IZI.

A cross-bar iS'I is welded between arms MI and 253 above shaft I53, Tocross-bar I3i, adjacent the center thereof, are welded'spaced,horizontally extending arms I83 and I35. These arms are supported on lid3!. Adjacent the other end of the arms E33 and I85 is welded a cross-barI81 and the ends of the arms have holesbored therethrough to receive apin or shaftliifi.

A'second circular lid or cover IiiI made of silicon carbide about teninches in diameter and one and one-half inch thick is mounted overtheopening 32 in the lid 3!. The cover I9I has a small opening I93centrally thereof in orderto permitobservation of the crucible contents,furnace flame and temperature without opening the cover. The cover iQIis rotatably mounted by means of arms I35 and I91 on shaft I89. The armsare welded to a steel band I99 surrounding the cover Hill. A steel rod2M is welded to the band I99 at the front end in order to permit thecover ISI to be swung open.

A short hollow stem 203 is welded to thesteel band I22 at the forwardend of thecover 3| in order to permit a .bar to be inserted therein whenit is desired to roll the cover and carriage back,- ward on the tracks.

Referring more particularly to Figure 5, the bottom "edge of tracks I29and I3I have an arcua'te cut-out 205 conforming to the periphery of thewheels I41 and I5I and is adapted. to receive these wheels. The notchesor cut-outs 205 are so located that when the wheels I41 and I5I areseated therein the arms MI and I43 tip forward and permit the lid3Itoclose over the top of the crucible chamber as shown in Figure 2.Vlfhen it is desired to open the crucible chamber a rod is inserted instern 203 and the front end of thelid M is lifted until rods I4! and I43are in substantially vertical position and the carriage isthen pushedrearwardly. As soon as the wheels I41 and I 5I engage the straightunder-side of the track, the lid 3I will be held in a suspended, openposition.

It will be understood the dimensions have been givenin order to furnishsome idea of the relative sizes of the various parts of the furnace.

Although the figures given are typical of commercial crucible furnaces,they may be larger or smaller as necessity dictates.

a To operate the furnace, the covers 3| and IQI are opened and thecrucible placed on the crucible block. The crucible is filled with thedesiredmetal or alloy in solid form and the covers closed. The pump forpumping oil into tank 91 and the air blower which forces air into inlet63 are started, the valves controlling the air and oil at the burnernozzle are opened and the oil issuing from the burner nozzle ignited.When the initial charge of metal or alloy is melted, the cover I-9I isopened and additional metal is charged to the crucible until thecrucible is full. The cover is closed and the crucible and contentsheated until they reach a temperature suitable for pouring, whereuponthe cover 3 I' is lifted upwardly and the carriage pushed backwardlyuntil the cover clears the top of the crucible chamber. The crucible iswithdrawn, the contents poured'and the crucible recharged for the nextmelt.

Y 'In a typical operation in which red brass was melted, from 31 to 34minutes were required to melt a charge weighing 210 pounds and bring itto pouring temperature, whereas with crucible furnaces now in generaluse, from 'to minutes are required. The gases in the crucible chamberattained'a temperature of approximately 2900-3100 F. and the metal waspoured at a temperature of 2300 F. The temperature of the gases enteringthe flue 69'was approximately 2900 F. and the temperature of thecombustion gases leaving the flue was approximately 2400 F. The air waspreheated to approximately 350 F. and the oil charged to the burner waspreheated to a temperature between 300 and 350 F. F'ourgallons of fueloil were consumed. The fuel'consumption represented a saving of 25% to30% overthat used in crucible furnaces of conventional type. I

- The figures given above, of course, will vary with different chargingmaterial. If desired, the oil may be preheated to a temperature betweenZOO-500 F. or more and the air may be preheated to temperatures between350-500 F. if desired. By preheating the oil to higher temperatureapproaching or above the flash point, instantaneous and substantiallycomplete complete combustion of the oil will take place in thecombustion chamber.

The tank 91 has been found to be essential if clogging of the oil lineis to be avoided. By providing a reservoir of relatively cool oil whichcan c'yclicly circulate between the tank and the heating coil 53,overheating of oil remaining in the coil when the burner is shut offbetween heats, with attendant carbonization and clogging of the coil isavoided. The air preheater structure, particularly the silicon carbidelining 89, remains intensely hot for some time after the burner is shutoff. If oil were permitted to remain stagnant in the coil 53, theintense heat of the preheater would crack and carbonize it. The tank,which may have a capacity of approximately two and one-half gallons fora furnace of the dimensions above given, contains a sufficient quantityof oil so that it is not raised between heats or at any other time, to atemperature at which the oil or other fluid fuel will crack.

It will be seen that I have succeeded in providing a furnace of noveldesign and construction which is capable of melting metals much morerapidly than was heretofore possible with crucible furnaces now in useand which effects a considerable saving in fuel. The construction alsofacilitates the charging and discharging of the furnace and a moreaccurate control of conditions within the furnace is possible by virtueof preheating and control of the fuel and air. Not only can temperatureconditions be controlled, but a neutral, reducing or oxidizingatmosphere can be maintained as desired.

I claim:

1. An integral crucible furnace comprising a crucible heating sectionand an air preheating section both of which are surrounded byinsulation, said crucible heating section having a heating chamber andsaid preheater section having a vertical annular closed air preheatingchamber, a combustion chamber in open communication with said heatingchamber, a burner extending into said combustion chamber, a passagewayfor combustion gases connecting the upper part of said heating chamberto the upper part of the central passageway of said air preheatingchamher, an inlet and outlet for air connected to said preheatingchamber, a closed duct connecting said outlet to said combustionchamber, an oil tank located in said air preheating section, a heatingcoil connected to said tank in such manner as to permit cycliccirculation of oil through the coil, said coil being located beneathsaid preheater chamber and in heat exchange relation with flue gasesleaving the furnace, and a conduit from said tank to said burner.

2. In connection with a crucible furnace having an open top heatingchamber, a pair of tracks mounted on said furnace, a carriage slidablymounted on said tracks, said carriage comprising a pair of upstandingarms having wheels rotatably mounted thereon which engage both lower andupper edges of said tracks, recesses in said tracks adapted to cooperatewith said wheels, a cover for said chamber rigidly mounted on saidcarriage and adapted to be held normally in open position and to besupported in closed position when said wheels engage said recesses.

3. The structure in accordance with claim 2 in which the cover has anopening centrally thereof, a second smaller cover adapted to close saidopening, said second cover being pivotally connected to said carriage sothat the cover can be swung open.

4. In a method of heating a furnace, the steps which comprise chargingpreheated fluid fuel to a burner in the furnace and there igniting thefuel, preheating the fuel by means of waste heat in the combustion gasesresulting from burning the fuel, said combustion gases being ofsuffioiently high temperature to carbonize the fuel if it were allowedto remain static in the preheater and cyclicly circulating the preheatedfuel to a body of relatively cool fuel and from said body back to saidpreheater when the burner is shut oil in order to prevent carbonizationof the fuel in the preheater by means of the hot surfaces contiguousthereto.

5. The steps in accordance with claim 4 in which the fuel is fed fromsaid body to said burner.

6. A crucible furnace comprising a crucible heating chamber, acombustion chamber in open communication therewith, a burner located insaid combustion chamber, an air preheater having an inlet and outlet, aclosed duct connecting said outlet to said combustion zone, means forpassing combustion gases from said crucible chamber into heat exchangecontact with said air preheater, a heating coil arranged in heatexchange relation with said combustion gases leaving said cruciblechamber, a tank connected at opposite ends to said heating coil and aconduit connecting said tank to said burner.

'7. In a crucible furnace comprising a crucible heating section and anair preheating section, means for internally heating said crucibleheating chamber and means for feeding preheated air to said heatingmeans, a passageway connecting the upper portion of said crucibleheating chamber to said air preheating chamber, the height of saidpassageway at its juncture with said crucible heating chamber beingrelatively small and gradually increasing in height toward its juncturewith said air preheating section, but the cross-sectional area along theentire length thereof being substantially the same.

8. In connection with a crucible furnace having an open top heatingchamber, the structure comprising a horizontal track, a carriageslidably supported on said track by means of a pair of spaced membersone of which is adapted to engage the top surface of said track and theother of which is adapted to engage the bottom surface of said track, acover for said chamber rigidly mounted on said carriage, said trackbeing of such varying width and said members being spaced apart such adistance that when the cover is directly over the opening of saidchamber, the carriage will be in such position that the cover rests onthe top of said chamber, but when the carriage is pushed back along thetrack so as to expose the opening of the chamber, the carriage will holdthe cover in suspended position.

9. The structure in accordance with claim 8 comprising a plurality ofsaid tracks upon which the carriage is adapted to slide by means of aplurality of pairs of said spaced members.

10. In connection with a crucible furnace having an open top heatingchamber, the structure comprising a horizontal track supported by saidfurnace, a carriage slidably mounted on said track by means of a pair ofspaced wheels, one of said wheels being adapted to ride on the bottomsurface and the other Wheel being adapted to ride on the top surface ofsaid track, a cover for said chamber rigidly mounted on said carriage,said track being of such varying width and said wheels being spacedapart such a distance that when the cover is directly over the openingof said chamber, the carriages will be in such position that the coverwill rest on the top of said chamher, but when the carriage is pushedback along the track so as to expose the opening of the chamber, thecarriage will hold the cover in suspended position.

11. The structure in accordance with claim 10 including a plurality ofsaid horizontal tracks, on which the carriage is adapted to slide bymeans of a plurality of pairs of said spaced wheels.

12. In connection with a crucible furnace having an open top heatingchamber, the structure which comprises a horizontal track mounted onsaid furnace, a carriage slidably mounted on said track by means of apair of spaced wheels one of which is adapted to ride along the bottomsurface and the other along the top surface of said track, a cover forsaid chamber rigidly fastened to said carriage, said track being of suchwidth and said wheels being spaced apart such a distance that when thecarriage is in such position that the cover is not directly over the topof said chamber, the carriage holds the cover in suspended position, arecess in a surface of the track into which the wheel adapted to rollalong said last mentioned surface is adapted to fit, said recess beingin such a position and of such depth that when said wheel is recessedtherein said cover is directly over said heating chamber and thecarriage is in such position that said cover rests on the top of saidchamber.

13. The structure in accordance with claim 12 comprising a plurality ofsaid tracks on which the carriage is adapted to slide by means of aplurality of pairs of said spaced wheels.

14. A furnace comprising a heating chamber, a burner connected to saidheating chamber, a source of liquid fuel supply connected to saidburner, a preheating coil located in a portion of said furnace which isadapted to be heated to high temperatures by means of waste combustiongases from said chamber, an auxiliary reservoir for said fuel, means forcontinuously circulating fuel from said reservoir to and from saidpreheating means, when said burner is shut off and while said portion ofthe furnace in which the preheating means is located is hot, and meansfor feeding preheated fuel to said burner.

KENNETH DAVID HOKE.

